The way Value.AuxInt represents unsigned numbers is currently
documented in genericOps.go, which is not the most obvious place for
it. Move that documentation to Value.AuxInt. Furthermore, to make it
harder to use incorrectly, introduce a Value.AuxUnsigned accessor that
returns the zero-extended value of Value.AuxInt.

Change-Id: I85030c3c68761404058a430e0b1c7464591b2f42
Reviewed-on: https://go-review.googlesource.com/102597
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>